1. Field of the Invention
The present invention is directed to an ultrasound apparatus, and in particular to a digital input stage for an ultrasound apparatus which processes signals for the purpose of ultrasound imaging as well as processing Doppler signals.
2. Description of the Prior Art
Ultrasound devices are known having a digital input stage which includes a processing circuit having a number of channels corresponding to the number of transducers in a signal-generating and signal-receiving transducer array. Each channel in the processing circuit concludes with an analog-to-digital converter.
When high quality B-scanners operate with ultrasound frequencies up to 10 MHz, and when such scanners have a number of complicated scan forms, it is necessary to undertake many finely-graduated switching events for the ultrasound beam control and focusing. Digital processing of the radio-frequency signals is suitable for this purpose. The use of digital technology for processing Doppler signals, however, is conventionally considered unsuitable, due to the inability of the analog-to-digital conversion to achieve the necessary signal dynamics which are needed to process echoes of permanent objects simultaneously with moving objects, for example, vein wall regions simultaneously acquired with the Doppler signal. It is preferable, however, that imaging systems be capable of also acquiring Doppler signals, at least as a option. Commercially available high performance equipment having a correspondingly low ultrasound frequency of, for example, 5 MHz includes input stages for Doppler signals which operate in analog fashion. The exclusive use of analog technology for generating the image, however, will not be retained in future devices because the aforementioned nominal frequencies up to 10 MHz can be achieved only with difficultly in analog technology.
The characteristics of Doppler signals of blood corpuscles are analyzed in an article by D. Hassler entitled "Beitrag zur Systemtheorie der Ultraschall-Impuls-Doppler-Technik zur Blutstroemungsmessung" from the publication Ultraschall in der Medizin, No. 8, 1987 at pages 102-107, 152-154 and 192-196. As reported in that article, the blood corpuscles (moving targets) from the small signal collecting area of a vein have a maximum dynamic of 30 through 40 dB, and typically lie 30 through 40 dB below the amplitude of permanent echoes from stationary tissue. Due to the limited topical resolution, it is unavoidable, especially given a signal collecting area close to the vein wall, that permanent echoes and moving target echoes will simultaneously appear. The moving target echoes can be discriminated from the permanent echoes on the basis of the frequency shift of the moving target echoes. A filtering method for separating the signals on this basis must therefore be used. This assumes, however, that the linear superposition of the two components is preserved in the signal flow chain preceding the filtering. Signal mixing at non-linear segments must be avoided.
It is known to acquire Doppler signals from an ultrasound applicator, in addition to the ultrasound signals, from European application 0 010 304. This is accomplished in this published application using mechanical scanners or electronic scanners with analog signal processing for sweep and/or focusing, given observation of the required signal-to-noise ratio. An example of a conventional digital RF input stage (front end) for a B-scanner is described in European application 0 170 072. Due to the aforementioned demands which are necessary to process signals having high dynamics at a high scan rate, hitherto unresolved technical problems have prevented the expansion of a conventional device of this type to also undertake Doppler signal processing.
It is generally believed, however, that digitally operating input stages represent the technically optimum solution for high quality scanners. In high performance equipment, one expects the ability to operate using ultrasound nominal frequencies between 2 and 10 MHz, and the ability to conduct a number of scan forms such as, for example, electronic sector scan, trapezoid scan and parallel scan without projecting edge length. These demands require a large range for the signal delay with extremely finely graduated variability. The most developed technique for meeting these demands is adjustment of the digital delay of the RF signals.
It is typical in such devices to use sixty-four input channels, thus requiring sixty-four analog-to-digital converters with eight bits, with a scan rate having a maximum of 40 MHz. This is sufficient for the image acquisition. To avoid the aforementioned non-linear characteristic, a bit width of at least fourteen bits is necessary in Doppler signal processing, because 30 through 40 dB additional dynamics are present which require an additional six bits. The current technical limit, however, is at twelve bits, given a scan rate of 20 MHz.